1. Field of the Invention
The present invention relates to both apparatus for a custody transfer metering system that utilizes a Coriolis mass flow rate meter and provides accurate totalized mass flow measurements and substantial fault detection capability, and various methods for use therein.
2. Description of the Prior Art
Frequently, whenever a product is being transferred in bulk between two parties, such as in a custody transfer, both parties measure the product to ensure that the proper amount is being transferred and that none of the product has been lost in transit between the parties.
For example, if a rail tank car or container ship load of a liquid commodity, such as oil, is being transferred between two parties, the oil is first passed through a meter, typically a volumetric totalizing flow meter, located at a loading station to ensure that the tank car is filled with the correct amount of oil. After the tank car or ship has reached its destination, the oil is pumped therefrom and then passed through a second flow meter, generally another volumetric totalizing meter, to measure the quantity of oil being received. The total produced by the second flow meter is then compared against the amount of oil that has been shipped to detect any loss that occurred in transit and/or in loading. Similar custody transfers occur when oil or another fluid is moved through a pipeline between two points.
Unfortunately, the density of many products varies with temperature. Inasmuch as the mass of a substance always remains constant, the volume of the substance will vary inversely with temperature. Therefore, temperature variations will disadvantageously cause discrepancies to occur between the totalized volumetric readings taken from both flow meters. In recognition of this, the temperature and density of the product is measured at both the origin and destination points and the totalized volumetric readings are appropriately converted at both points into totalized mass readings. Frequently, this conversion is performed manually using tabulated data. Alternatively, sufficient capability is often incorporated into the flow meters to automatically measure temperature and density of the product and thereafter perform the required conversion to provide mass readings. Unfortunately, this conversion, whether performed manually or automatically, is rather inaccurate and often injects error into the mass readings. Therefore, the art has turned to meters that measure mass directly.
Unfortunately, many commercially available mass flow meters process incoming signals using analog circuitry. Unfortunately, analog circuit components are susceptible to offsets. Moreover, electrical characteristics of these components often change due to drift, aging and temperature variations. As such, all of these factors disadvantageously inject a unacceptable degree of long term inaccuracy and instability into the performance of the flow meter.
Furthermore, various government agencies in many countries, such as PTB located in the Federal Republic of Germany, strictly regulate the performance of metering devices that are used for custody transfers occurring within that country. These regulations not only govern accuracy of the device, but also govern access and importantly fault detection. To obtain government approval, such a metering device must first exhibit a certain minimum accuracy. Second, such devices must be substantially tamper-proof and thereby frustrate attempts made by unauthorized persons to gain access to the device in order to change a metered reading and/or a totalized quantity. Third, the metering device must possess the capability to detect faults that might occur in its performance, stop totalizing at the occurrence of the fault and thereafter provide an indication to the user that a fault has occurred. In this manner, the user is alerted to ignore the reading. In fact, the German agency PTB, requires that such a meter must be able to detect at least 95% of the possible single faults that could occur before the meter can be approved for use in custody transfer applications in the Federal Republic of Germany. While many presently available commercial flow meters possess requisite accuracy and are sufficiently tamper-proof, they do not detect single faults at all. Once a fault occurs, these meters simply ignore the fault and continue to totalize until someone happens to subsequently discover that the meter is operating erroneously and then takes the meter out of service. Other commercially available flow meters that do possess fault detection are generally incapable of detecting single faults to the 95% fault level. Consequently, to the extent that any commercially available flow meter is unable to detect a single fault or can not do so to the 95% level, then such a meter simply could not be used in custody transfer applications, particularly in the Federal Republic of Germany.
Therefore, a need exists in the art for a highly accurate and stable custody transfer metering system that provides direct mass flow measurements and is also substantially tamper-proof and has substantial single fault detection capability.